ORTMANN: monograph of the naiades of PENNSYLVANIA. 69 



mentions at least one place in western Missouri (Little Blue River, Courtney, 

 Jackson Co., near Kansas City). The species has never been reported from the 

 region of the lower xAlississippi, and although Call (1896a and 1900) includes the 

 Alabama River in its range, this record has not been confirmed by anybody else 

 (See Lewis, 1877). 



Genus Pleurobema Rafinesque (1820). 

 Ortmann, 1912, p. 261; Simpson, 1914, p. 732. 



Type Unio clava Lamarck. 



The conception of this genus has been changed greatly in recent times. In 

 Pennsylvania, I distinguish two species, but one of them is quite polymorphous, 

 with a number (five) of variations and ecological (or local) races or varieties. 



Key to the Species and Varieties of Pleurobejl\. 

 cti. Shell rather hirge, upright, or oblique. Epidermis chestnut to dark brown or blackish, with in- 

 distinct capillary rays, which generally are not broken up into blotches. 

 hi. Shell not very oblique, more or less upright. Nacre white or red. Radial furrow strong, weak, 

 or absent. Diameter variable. 

 Ci. Furrow strong or weak, but always present. Nacre white, very rarely red. Diameter 

 mostly over fifty percent of length (very rarely less). Shape upright. 



di. Outline subtriangular; furroj? strong p. obliquum. 



(h. Outline rounded; furrow generally weak, but present P. obliquum cordaium. 



c.. Furrow generally absent (rarely a trace seen). Nacre white or red. Diameter variable. 

 Shape subtriangular, upright, or slightly oWique. 

 di. Shell rather large. Growth lines irregular and indistinct. Epidermis dark, 

 e,. Nacre white, rarely red. Diameter fifty percent of length or more. 



P. ohliquum catUlus. 

 C2. Nacre white, salmon, or red. Diameter less than fifty percent of length. 



P. ohliquuin coccineum. 

 dn. Shell smaller. Growth lines more distinct and regular. Epidermis lighter. 



P. obliquum pauperculum. 

 62. Shell very oblique, beaks dii'ected anteriorlv. Nacre generally red, rarely wliite. Furrow strong, 



rarely weak. Diameter mostly over fifty percent of length P. obliquum rubrum. 



oo. Shell rather small, very oblique, beaks located weU anteriorly. Epidermis yellowish or light brown, 

 with distinct rays, which are often broken up into blotches p. dava. 



Pleurobema obliquum (Lamarck) (1819). 

 Quadrula obliqua (Lamarck) Simpson, 1914, p. 881. 



Plate VI, figs. 4, 5, 8. 

 Records from Pennsylvania: 



Call, 1885 (Allegheny River). 

 Harn, 1S91 (western Pennsylvania). 



